At the Edge of Time: Exploring the Mysteries of Our Universe’s First Seconds (Science Essentials Book 32)

by Dan Hooper

But reaching even farther back in time—into the first seconds and fractions of a second after the Big Bang—we transition from having incomplete information to having essentially no direct observations on which we can confidently rely. This era remains hidden from our view, buried beneath as-yet-impenetrable layers of energy, distance, and time. Our understanding of this period of cosmic history is, in many respects, little more than an informed guess, based on inference and extrapolation. Yet it is clear that these first moments are the key to many of our most urgent and enduring cosmic mysteries. Understanding this era is essential to understanding our universe.

The possibility that a single object could be both a particle and a wave led to many strange and counterintuitive conclusions and was inconceivable before the paradigm shift we now call quantum mechanics. The answer to the question of whether light is made up of particles or waves turns out to be none of the above, and both of the above, at the same time. From the vantage point of the old thinking, a new paradigm can seem ridiculous and utterly absurd. It reminds me of when Bob Dylan sings, “The sun’s not yellow, it’s chicken.” To make sense of it, you have to forget something about what you think the words mean, and only then begin to build up a new way of understanding the ideas involved.

Laws of physics are laws of motion. Without space and time, we can’t talk about what it means for something to be moving, or for something to be close to or far from something else. We can’t talk about something happening without a notion of time for it to be happening in. Even the concept of energy is built upon space and time, because energy is ultimately nothing more than motion or the potential for motion. Without time and space, nothing can change, and without change, it is hard to imagine any reality worth imagining. Without time, nothing happens. Without space, nothing is.

Regardless of Einstein’s own philosophical objections, Friedmann was beginning to show that the theory of general relativity does not allow for the possibility of an unchanging universe. Change is simply inevitable.

So distances in space are dependent on what we use to measure them. As a consequence, we have at least two very different ways that we can choose to think about expanding space. The first, and more conventional, of these is to think of the amount of space in our universe as increasing. But alternatively, we could instead imagine that everything within space is shrinking, while space itself remains unchanged. Both ways of thinking about this are completely equivalent and indistinguishable from each other.

individual photons are stretched by the expansion of space, which increases their wavelength and reduces their energy and temperature.

As a consequence, whatever the ratio of light to matter is in our universe now, we know that it must have been higher in the past. And although our universe contains much more energy in the form of matter than in light today, if you look back far enough in time, you will find an era in which this was not the case. Our universe began not in a state of matter, but in a state of light.

The temperature of this radiation—known as the cosmic microwave background—is currently a frigid 2.728 degrees above absolute zero, or about 455 degrees below zero Fahrenheit.

The formation of the first atoms was the key event in cosmic history as far as observational cosmologists are concerned. Before the electrons and protons had become bound together into atoms, all of space was filled with a plasma of electrically charged particles—a plasma that was almost entirely opaque to light. For this reason, our telescopes can observe only the light that originated after this key event. Using a telescope to look farther back in time is as difficult as using one to look deep beneath the surface of the Sun.

And yet, despite all of this progress, many important questions remain unanswered—in particular when it comes to our universe’s earliest moments. We still don’t know, for example, how the protons and neutrons that make up the atoms in our universe survived the heat of the Big Bang. From what we do know, it seems that these particles should have gone extinct long before a single atom had had the chance to form. But something that we don’t yet understand must have prevented this from taking place.

This makes the LHC the largest cryogenic environment in the world. In fact, we know of no region in the universe that is both so large and so cold. Even the vacuum of outer space is warmer in comparison.

Our prehistoric ancestors didn’t hunt with electrons, and we didn’t grow up playing with quarks.

But even at low temperatures, electrons can radiate and create new particles of light—photons. Thus a box that starts out containing only electrons will, over time, evolve into a box of both electrons and photons. The reason that the number of electrons stays fixed in this case is that each possesses electric charge, and, as far as we know, electric charge can never be created or destroyed. If the number of electrons in the box were to increase or decrease, the total amount of electric charge would change too. Photons, in contrast, carry no such electric charge, and thus can be freely created or destroyed.

Every kind of particle in nature has an antimatter counterpart—a version of itself with the same amount of mass, but with opposite values for its electric charge and other quantum properties. The proton has the antiproton, the muon the antimuon, and so on. The only exceptions are those particles with no quantum properties to change the sign of. The photon, for example, has no electric charge or other such quantities. For this reason, the antimatter counterpart of the photon is simply the photon itself.

The conservation of charge requires only that the number of electrons minus the number of positrons stays fixed. So an electron can be freely created or destroyed, but only if a positron is created at the same time. And electrons and positrons are not alone in this respect. If we turn up the temperature of our box even more, other kinds of particles will begin to be created—starting with pairs of muons and antimuons and working up to Higgs bosons and top quarks.

Whereas the electric charge of a particle causes it to feel the effects of the electromagnetic force, a particle’s color enables it to experience the strong nuclear force—the force that binds quarks together to form protons and neutrons, and that holds protons and neutrons together inside of atomic nuclei.

As space expanded and cooled, some types of particles began to disappear from our universe. The first of the known particles to become scarce was the top quark—a particle that was discovered in 1995 at the Fermi National Accelerator Laboratory,

Roughly 100 trillion neutrinos pass through your body every second of every day, most of which were produced in the Sun as a by-product of nuclear fusion. These ultra-light particles have virtually no impact on your body or your health, because they interact so rarely. In fact, these particles can pass through the entire Earth without any appreciable effect. To neutrinos, it is almost as if matter doesn’t exist.

there are few questions about our universe that are more perplexing than how and why the fundamental constituents of atoms—quarks and electrons—came to exist.

We know how to calculate how many quarks were present in the early universe and how this quantity changed as our universe expanded and cooled. We can also calculate when these quarks coalesced in groups of three, to form the first protons and neutrons. This should not be a hard problem. Yet it is. When we carry out these calculations, we find that they simply do not predict that a universe like ours—one full of atoms—should have emerged from the Big Bang. Instead, they tell us that very few quarks and very few electrons should have survived these first moments. According to the math, the world should be virtually free of atoms.

something must have happened in that primordial soup that caused the total amount of matter present to become slightly greater than the total amount of antimatter. Even a very small imbalance, if established early enough in our universe’s history, could explain the world we see all around us today. A primordial asymmetry as slight as 10 billion and 1 particles of matter for every 10 billion particles of antimatter would be enough to enable a small but adequate fraction of the matter to survive and escape the early universe intact.

Antimatter has long fascinated and confounded cosmologists. Everything we know about this mysterious stuff places it on an equal footing with matter. Antiparticles possess the same mass, are created and destroyed in the same ways, undergo the same interactions, and otherwise represent seemingly perfect—and yet opposite—copies of their particle counterparts. It is the fact that these substances are so precisely alike that makes it so hard for us to understand why there is so much more of one than the other in our universe today.

And the light generated from an antimatter star or an antimatter galaxy would be entirely indistinguishable from that produced by any ordinary star or galaxy.2 The next time you look upon the night sky, you might wonder whether some of that light might have been generated in the nuclear furnace of an antimatter star. In perfect isolation, there would be no way to know for sure.

The fact that there is far more matter than antimatter in our universe flies in the face of everything physicists have learned from accelerator experiments about how both matter and antimatter behave. For reasons that we do not yet understand, these substances managed to avoid their mutual annihilation in the early universe. Somehow, much more matter than antimatter survived the first moments that followed the Big Bang.

Having been sequestered from the physics community for two decades, he didn’t have the same preconceptions about what he should or shouldn’t be interested in or be working on. As a result, Sakharov was able to produce and publish papers on the new theory of the strong nuclear force—known as quantum chromodynamics—as well as on general relativity, and on how distributions of matter evolve as space expands. He wasn’t a particle physicist, and he wasn’t a cosmologist. It was precisely this that made it possible for him to become both.

In a short paper published in 1967, Sakharov laid out three conditions that he demonstrated must be met in order for a perfectly balanced collection of matter and antimatter to become dominated by matter. Although they can be complicated to describe in their full technical glory, these conditions can be paraphrased roughly as follows: There must exist some kind of interaction between particles that can change the total number of quarks minus the total number of antiquarks. Nature must have a bias in these interactions that favors the creation of matter over antimatter, or the destruction of antimatter over matter. These interactions must have had an opportunity to act under particularly violent or rapidly changing conditions at some point in the early universe.

Physicists now use the word baryogenesis—literally the origin of baryons, which are made of quarks—to refer to the transition from a balanced state of matter and antimatter to one dominated by matter. Today, we still don’t know exactly how baryogenesis took place in our universe. But we do know that the three conditions laid out by Sakharov half a century ago were somehow satisfied.

For one thing, if quarks can be destroyed without destroying antiquarks, then it should also be possible for particles such as protons—which are made of quarks—to decay. In other words, Sakharov’s first condition implies that every atom in our universe is ultimately unstable—at least slightly. Even atoms cannot last forever.

“no-hair theorem.” In a nutshell, this theorem says that you can completely describe a given black hole by only three quantities: its mass, its electric charge, and its rate of rotation. This means that any two black holes with the same mass, charge, and spin are completely identical in every respect. From this it follows that an electrically neutral black hole that forms out of matter is exactly the same as one that forms out of antimatter. The black hole’s gravity essentially erases the net number of quarks that went into its formation. And although physicists still debate the full consequences of this theorem, there is broad agreement that it provides support for the idea that nature satisfies the first of Sakharov’s three conditions.

This phenomenon was observed for the first time among particles called kaons—states consisting of a strange quark and a down antiquark bound together. It was previously known that the weak force could transform a kaon into an antikaon, and vice versa. The new fact that these experiments revealed was that these processes do not happen with the same likelihood in both directions. There is, in fact, a bias between matter and antimatter built into nature’s very structure. The violation of CP symmetry is exactly what is needed to satisfy Sakharov’s second condition, and this makes it is possible—at least in principle—for processes that favor matter to win out over those favoring antimatter.

Unlike the arc of the baseball—which looks the same whether moving forward or backward through time—the weak force acts differently in these two temporal directions.

The forces that act among atoms cause them to resist compression—if you don’t believe me, just try squeezing a balloon. But dark matter is different. Unlike atoms, it is effectively immune to such forces, allowing gravity to compress it much more quickly, and shaping it into the scaffolding that the rest of our universe’s structure would later be built upon. Long before there were any galaxies, the dark matter began to gather together into enormous clouds. It was the gravity of these dark clouds that attracted and pulled together the atoms that would ultimately go on to form galaxies themselves.

For a small fraction of the astronomers and physicists who have studied this problem, the evidence for dark matter has not proven entirely persuasive. To these scientists, dark matter is not the only way that the motions of stars around galaxies could be explained—or even the patterns of galaxies and galaxy clusters found throughout our universe. Rather than postulating one or more new forms of nonluminous matter, they speculate that we may instead have misunderstood the effects of gravity.

The key finding of this paper was instantly apparent to anyone who compared these two maps: whereas the bulk of the visible matter in this system—the hot gas—was confined largely to the innermost volume of these clusters, the center of gravity was not. For the first time, astronomers had found a system in which the source of the gravity—and thus dark matter—was not centered around the same place as the visible matter. No MOND theory predicted anything like this. Knowing this, the authors of this paper gave it the title “A Direct Empirical Proof of the Existence of Dark Matter.”

The moral of this calculation, at least as it was presented to me in the early 2000s, was that dark matter—whatever it is—is likely to be made up of particles that interact predominantly through the weak force. We called this class of dark matter candidates weakly interacting massive particles, or WIMPs. Although many kinds of WIMPs had been proposed over the years, none had been directly detected in any experiment. The closest things to a WIMP that we had ever observed were the neutrinos, but these particles are far too light and fast moving to constitute much of our universe’s dark matter.

If something caused the early universe to have expanded differently than we currently think it did, for example, then the results of this calculation could come out very differently. Alternatively, if the dark matter is extremely feebly interacting—even more so than WIMPs—then these particles may not have interacted often enough to have ever reached equilibrium in the first place, causing this entire line of reasoning to fall apart.

Although dark matter is constantly passing through all of us, we are also immersed in a violent sea of ordinary atoms and radiation. In order to isolate and identify the occasional push from a dark matter particle, one must first quiet the surrounding sea of atoms. The universe, after all, is a very noisy and volatile place.

Supersymmetry hypothesizes that there is a close connection between two different groups of particles—fermions and bosons. Fermions include all of the leptons and all of the quarks, while the photon, gluon, W, Z, and Higgs are each bosons. The basic idea behind supersymmetry is that each fermion is directly related to and partnered with a boson with many of the same properties, and vice versa. For example, in order for the electron to exist, there must also be a bosonic superpartner with the same electric charge and other quantum properties—the selectron. And because the photon exists, supersymmetry posits that nature must also include its fermionic counterpart—the photino.

For one thing, the mathematical structure of supersymmetry is remarkably deep and beautiful. Though many people are surprised to learn it, aesthetic considerations have often played an important guiding role in theoretical physics.

A decade ago, we thought of dark matter as weakly interacting. Today, we are asking whether dark matter interacts with the visible world at all.

You could even imagine that the dark matter might not be alone, but might instead be only one of many forms of matter that almost never interact with any of the known particles. These elusive particles would constitute what physicists call a hidden sector, which could evolve and interact in any number of potentially complex ways.

The universe we live in and experience has little in common with either of these extreme eras. Our world, it seems, is in a moment of calm between two incredible storms. All indications are that our universe both emerged from, and will likely return to, a very different cosmic state.

In effect, the expansion of our universe causes all points in space to be surrounded by an impenetrable horizon, beyond which nothing can be observed and no communications can reach. In the current epoch, our cosmic horizon is the surface of a sphere with a radius of about 46.5 billion light years, with us at the center.1 This sphere is, for all practical purposes, our universe. Its volume contains all of the space that we can see, study, witness, experience, communicate with, influence, or be influenced by. The boundary of this sphere is entirely impenetrable and forever will be—at least as long as the expansion rate of our universe doesn’t begin to slow down. Things beyond this distance are not merely obscured from our vision, but are utterly disconnected from us. All of space beyond this distance is forever lost from our world.

During the formation of the cosmic microwave background—about 380,000 years after the Big Bang—each point in space was surrounded by a horizon about a million light years in radius. From our vantage point today, a region of space this size appears to be about one degree in diameter. This means that the background of microwave radiation that we observe across our sky does not originate from one causally connected region, but from many thousands of independent regions that had absolutely no way of influencing each other. But without any way to interact with each other, how did these regions come to have temperatures that are so nearly identical? To return to my earlier analogy, it is as if we found a collection of thousands of nearly perfectly synchronized stopwatches, with no explanation for how they came to be that way in the first place.

Furthermore, inflation predicts that the primordial variations in density should be approximately—but not perfectly—“scale invariant,” meaning that the pattern of density variations across a given region will look roughly the same as the pattern you would find if you were to zoom in on the image. In this sense, inflation predicts that our universe should be similar to what mathematicians call a fractal. Cosmologists quantify this property using something known as the spectral index of primordial fluctuations, which most theories of inflation predict should be just a little less than 1 (where 1 is the value for a perfectly scale-invariant pattern). The most recent measurements of the cosmic microwave background place the actual value of this quantity at about 0.965—entirely consistent with what inflation had long predicted.

If our universe never underwent an era of inflation, one is forced to ask how it came to be both so flat and so uniform. If we were to reject inflation one day, we would still need answers to these perplexing questions. If not inflation, then what left our universe in such a strange and specific state?

More recent models sometimes involve extra dimensions of space, beyond the three that we know and experience. Within these theories, the forms of matter that we experience are confined to what is known as a brane—a three-dimensional space that is, in reality, a subset of the greater multidimensional space. Our world appears to us to be three-dimensional solely because electrons, photons, and the other known forms of matter can travel only within the brane’s limits. The brane itself, however, can move through the full dimensionality of space. Within this kind of picture, something that looks like a Big Crunch followed by a Big Bang could occur whenever our brane collides with another.

Consider the well-established fact that our universe is expanding. Because of this expansion, every point in space is surrounded by a cosmic horizon, beyond which one could never travel—even in principle. We will never be able to observe or interact with anything that lies beyond our horizon, and someone beyond our horizon could never observe or interact with us. As far as I’m concerned, a place that one could absolutely never observe, interact with, or travel to is not in the same universe. Although there are many different definitions of this word, I think of a universe as a region of space in which things could in principle interact.

That is the thing about infinity: it takes things that are otherwise very unlikely and makes them all inevitable.

The expansion of space divides it into a number of causally disconnected regions. For all intents and purposes, these regions are not part of a single universe; rather, each is a universe of its own. Each point in space is surrounded by an impenetrable cosmic horizon, the size of which is determined by how fast space is expanding. The faster that space is expanding, the closer this cosmic horizon will be to the point that it surrounds. During eras of accelerating expansion—such as our current era—space is continuously being divided into a larger number of disconnected universes. And while this is taking place to some degree in our universe today, there was a point in cosmic history during which the accelerating expansion of space was far more dramatic. Nothing created a multitude of universes faster than cosmic inflation.